Automatic refrigerating defrosting control



Feb. 23, 1954 MINORU FUJH 2,669,848

AUTOMATIC REFRIGERATING DEFROSTING CONTROL Filed Oct. 21,. 1952 INVENTOR F5 av T One cycle Q BY 0 ATTORNEY am 000 ,ruull Patented Feb. 23, 1954 AUTOMATIC REFRIGERATIN G DEFROSTIN G CONTROL t MinoruFujii, Washington, D'. CL Application ()ctoberzl, 1952',,'Serial. NOL 31.5;917

'Il'iisn invention? relates-,- to automatim control means for defrosting the cooling. elementof-auto matic". refrigerators; and: has for its primary object; the provision ofr improved means otsimple COIlStIllCtlOIlifirlIdiPOSitlVGi operation: for accurate-- 1y. proportioning-t thew defrosting: action in accordance: with: the" amount of" frost which: has accumulated so: as'vtotinsure; complete defrosting without:excessivesshutdown ortexcessiye: production of heat whi'chlwuuld impair" the: efficiency;

of operation" of. the: refrigerator unit;

An' automatic: defrosting": system: must include: some means for: detecting" an'l accumulation of frost at -w-l1icli it? is; desired to commencedefrosting; means for; initiating" a; defrosting: a0

tion #when this accumulation ofif'frost is: detected}. and means to end the defrosting" action: when. the ob'j ecthas been accomplished? It is known inthe art to detect f'rost accumulation by such devices as measurement of the I physical thicknessof ice' accumulation by a mech'anicallmeche anism; by detecting: the change in temperature; in terms of gas pressure; produced at a'= specified distance from the: cooling unit? due to the ac-' cumulation of ice reaching to that distance; by

detecting the interruption of: a beam of light produced byice accumulation: and by notingz the response to'increa'sed current flow to the motorof 'a fan which blows air a'cross #the evaporator: due toincreasedpressure drop caused by accumu lation' of ice. More' naive systems utilize a timerwhich initiates andends defrosting at arbitrarily predetermined time periods, or else count the number of" door openings.

necessary" that the defrosting action must be? ended when the' defrosti'ng hasbeensatisfactorily accomplished) Various methods have b'een resorted to in" orderto accomplish this, the simplest of which is" the useofa pre-set period of d frosting: While thisis generally satisfactory',

it" is; of course; not as efficient; especiallywhen operating temperature is changed; as a system whichaccurately determines: when the cooling-i unit hasf heen completely defrosted: and (initiates cessation of II defrosting? action:- at. that time:

The: defrosting; itself maybe-accomplished in:

various ways; Among'thesefaretthe flow 'of the'r hotigaswor'liquid through:the evaporator; thew use of a water: on other liquid sprayt'the: use

of two evaporators which: are alternately olefrosted; the? user ofi' an: electricit heater-i in consductiweizcontact withtthew;surfacettotith'eficooling1' unity: theiuseor hot 'a irectedioverlthetevapk orator'mnd the revers lflow of the refrigeranttfi wherein the r evaporator is used as a condenser. In addition to the above it is,-=of-course .alsc possible to merely stoprthe flow ofdrefrigerantp but this is slow and generally unsatisfactory Ithas-been foundthat the use of local heating applied to the coolingunit can quicklyand=satisfactorily accomplish defrosting withoutappre ciably interfering with the normal operation of: the refrigerator.

All of the prior methods of defrosting of=which I'am aware are subject to certain disadvantages: They either employ relatively complicated me chanieal parts which are a source: of trouble; or else are unduly expensive; or else will-be-ver i dilficult toadjust to variousoperatingiconditibnstu ifthey' are not-possessed of the required." degree? frost; both or all 1- of the: temperaturessensitite units will he at approximatelyrthe same tameperature; Or a predeterniinedf difference in termperature will I exist" due to: thc-ys diflferent distances elements are suitably' arranged in a bridge cir cuitso 'that; in effect; the difirenc'e's in tem pera'tures of i the elements are detected by the circuit; then it is possible i to" balance: the 'b'iidge circuit sothat the operation is not afiected normalfvariation iii-temperature of the airwith in the refrigeratori 'I'l'nat is, the refrigerator can be set tooperate' at various-temperatures without beingafiected by or affcting'tlie operation of th'e frost detecting elements? Hewever: when? frost-, formed around the cooling unit reaches one ofthe element's; its temperature will be drastically lowered" as compared to an! e ment set"v at a slightly greater!distanceffroirt :the: cooling unit which has not "yet'b'een reached by the: frost: formation. This; differencer' in: tein perature may then be? used to 'initiate the": d et frosting: action. The "0855313101110? defitostlng aL'Cftionamambe accomphshedzl-if desired: by; meanst i a mere'rsophisticated:system for endingtdefi b'stting is shown wherein the control is transferred to a second set of temperature-sensitive resistors after defrosting action has been initiated, and said second set of elements controls the defrosting accurately in accordance with actual conditions on the surface of the cooling unit.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment thereof as shown in the accompanying drawings, in which:

Fig. 1 is a schematic diagram showing the invention applied to an internal defrosting system;

Fig. 2 is a chart showing graphically the normal frosting and defrosting cycle of Fig. 1;

Fig. 3 is a schematic diagram showing the invention applied to an external defrosting system, and

Fig. 4 is a chart showing graphically the normal frosting and defrosting cycle of Fig. 3.

Referring to Fig. 1, adjacent to the evaporator unit or cooling unit 2 there are placed three temperature sensitive resistance elements a, b, and c, of which element a is placed very close to the cooling unit, or if desired, in contact therewith, and elements b and c are respectively spaced at increasing distances from the surface of the cooling unit 2. For this purpose, commercially available thermistors are used, although any suitable temperature sensitive resistance elements having a large temperature coefficient of resistivity may be used. These resistance elements are connected to a double Wheatstone bridge arrangement generally indicated at t, which is actually two separate Wheatstone bridges having some common elements and so connected by means of switches as will be described below that one or the other of them is in the circuit at any given time. The normal frosting and defrosting cycle of the refrigerator is indicated graphically in Fig. 2, wherein the ordinate represents current flow across the bridge as the latter becomes unbalanced, and the abscissa represents time. The circuit of Fig. 1 is shown in the normal condition during normal operation of the freezer which is arbitrarily represented as zero time in Fig. 2. At this time resistors b and c are connected in the circuit as can be seen by following lead 6 from thermistor b to contact 8 of stepping relay Hi, through switch arm l2 to arm 14 of the bridge. The bridge circuit continues through fixed resistance It and adjusting resistance l8 of a third arm of the bridge to contact 28 of switch arm 22 of stepping relay 0, thence to the fourth arm 24 of the bridge. Inserted in this fourth arm is thermistor c. It will thus be seen that with the arrangement shown thermistors b and c constitute two arms of the Wheatstone bridge, resistance l4 constituting a third arm and resistances l6 and I8 constituting a fourth arm. The input of the bridge is provided at terminals 26 by any suitable source, and the output of the bridge is carried by leads 28 and. 30 to a suitable amplifier 33, the amplified output being fed to stepping relays If] and 32. The usual thermostat for controlling the temperature of the interior of the refrigerator is indicated at 34. The operation of this thermostat controls solenoid actuated valve 36 or any other conventional elements whose operation controls the condition of refrigeration, such controlling circuits beingno part of the present invention. The thermostat also controls, through leads 38, relay 40 which in turn.

controls the power input to the Wheatstone bridge through terminals 25. The circuit of Fig. 1 is intended for use with an internal-type defroster system, that is, one wherein the defrosting operation is accomplished by heating the evaporator 2 from within. This may be accomplished by any of a number of Well known methods, one of which is to flow hot gas or liquid through the evaporator, while another is to reverse the flow of refrigerant so that the evaporator is used as a condenser, and still more common is the use of electric heating by thermal conduction. My invention is not concerned with the type of defrosting employed, but only with provision of a control system therefor. However, different circuits are required for internal defrosting as above described, and for external defrosting of the type wherein an external heating element near the evaporator is used to accomplish the defrosting. The arrangement of Fig. l is for defrosting control in the former instance, while Fig. 3 shows a control system for external defrosting.

Relay 4B is provided for control by thermostat 34 so that there will be no possibility of the Wheatstone bridge circuit going into operation to initiate defrosting while the thermostat is still calling for more cold, that is, for continued operation of the refrigerating system. Thus it will be seen that when the thermostat energizes solenoid 35 (or any other control device) to cause operation of the refrigeration system, relay 453 will also be actuated to hold open switch 4i and thus insure that the Wheatstone bridge circuit remains deenergized. During the normal operation of the refrigerator, as is well known, frost accumulates on the evaporating unit 2. However, as long as the frost accumulation does not actually contact element I), this element and element 0 will both be at a temperature substantially determined by conditions in the interior of the refrigerator. Element 1) will be a little cooler because it is closer to the evaporator unit 2. However, the resistance ll! of the bridge can be so adjusted that under these conditions of operation the bridge is balanced and stepping relays Hi and 32 remain in the condition shown in Fig. 1. stage I of Fig. 2. As the actual frost coating creeps closer and closer to thermistor b the temperature difference between E) and 0- increases continually, until when the coating actually contacts element 1), this temperature difference rises to such a point that the resultant unbalance of j the Wheatstone bridge due to the change of re- I sistance element 17 causes operation of stepping relays l0 and 32, marking the end of stage I and the beginning of stage II as shown in Fig. 2. this point, if thermostat 34 is calling for more cold, relay 4!! will hold switch ll open and nothing will happen. However, as soon as the thermostat deenergizes its outgoing circuits, switch li closes and the unbalanced Wheatstone bridge is energized to operate relays it and 32. Of course, if

the beginning of this unbalance found switch ii closed, the relays would be actuated immediately.

Operation of relays id and 32 will mov all of the switch arms affected by these relays from the upper contact position shown in Fig. 1 to the lower contact position. Relay 32 thereby cuts out thermostat 3d and actuates heater control 42 to initiate the internal defrosting operation.

Switch arm [2 of relay If! disconnects element b from thebridge. and connects instead-element a, the circuit beingfrom element a through lead 44, co tact 4 switch arm |2 ,to arm M of the This is the condition represented at bridge-z. At theasamertimer switch'rarmill 'toifreiatt I ll: disconnects; the; arm includingi resistances Id? and: land substitutes atherarxm ineludineamsiste ance 48;; andl variable: resistancezihasr. be; obvious.- fromwinspeationrof thezoirouitt. Thezbridaez: is now under control of elements .Gtsfllldl'OJiIlStBEdf. of: b,-.and.-.c; as heretofore, Since: thewrelays are notdireetional; .ita willrhe; understoodtthat. theiroperation will .be:.dependent only -uDmthewfllflQ5- nitudezoi. the; current ,flOWrdUefiO; :unbalancegand; therefore it does notzmatter; est-farms the relays! are concerned, which one of the elementsriswolder-anduwhich warmen Att the momenttofidefrosting, represented by: the? beginning: on stage II t of Fig. 1.; there will; of course; also; be: aylarger temperature; difference: between; elements; w. and: element 0; However; rheostat; th. ot the: bridge-=1 is. so adjusted that this. difference;doestnotzcausew sufficient unbalance: to operate, the;- relay Ase. heating; commences,-, the temperature-inside; of; evaporator 2- rises rapidly and element .mhecomes; warmed so thatthere is nowa temperaturadifference; in the, opposite. sense: from. before;. as. shown in stage II. However, as. long; as there is ice surrounding the evaporator unit; element, a. cannot rise in-temperatureto appintgsuflicient: 1y highto'cause operation-.ofrthe relays- Aften allof. the ice. is me1ted,.,the.temperature of..e1e;.-- ment a very rapidly rises, causinggsuflicientune balance of the bridge to operate. the, stepping relays to restorethe initial condition showning Fig. 1.. Thisis the beginning.of,stage;.III, which; is, of course, thesame; aststage ,I, and the cycle is .now repeated.

One, important advantage 1 of my. invention as, 3 exemplified @by. the above systemis thatthebridge. can be so adjusted that. its. operation. is. not-.- affected. by even a, fairly. radicahv changenint the operating temperature of, the, refrigerator; as, determined ,by the customary manuali setting. of; the thermostat .3f4.. Sincethe. operation...dep.ends... upon the. difference. in temperat.ure initially. in. elements b'andc, a change in..tlie.absoli1te,tem-. perature. of, the. interior, of f therefrigerator in which botnb andc. are located .will'fnot materially? affect this state or; balance;.and' 'it is notLuntiIZ theapproach of'actualfrosttoelement bfchanges its condition in' a; very'different manner from that of element c'tliatthe balance oftliekiride. is disturhedsufficiently, to cause operation. Of"

justed by means .ofjvariable resistances; 18"and 50, respectively, that normal. variations. in open? ating temperature will I not. cause. unwanted. 0neration of {the defroster unit It .will 1 bezseenwthat1theaaibove systemznrfidimes; defrosting. strictly. in accordancetwithtthetformaetion'; of. frosttup on' theaevaporatorr unit,=, andtten minates the defrostingwhenythe .;-iceahas .beeome completely. melted. The. heating operation; is

continued. forl-nowlongena periodwthamissessentiall Contact 1 the h to accomplish-the;desiredrresulti.yet rit issneyers terminated; while; there ierstillisifrostilett I011; time. evaporatormnitandithcn ob'zisthusmotzccmpietelw finished:

In" they casavwnerev am external-i heater: unituism i remain illnf hef down'zpositiom.

second; actuation-tot"; these. relays ,wto return them:

- ordinal-m type}; that is-7.; wli'em r additions :areznecessary: as ishown-inili i'g: 3 MostinrEiggiaaare-zvery similartto-those':

offtheaelements imEigr. 12a. andiane representedtby thesame refer emceechametersrwithiaal.primevaddedi Additionali relays-z A and i2? are, provided which are or the" they areenergized their? switch: arms are: drawni down: toward the coila and! when; deenergizedi' movee awayrfromi therein in'tthe case of relay: 50-; .armz-5l:willlmove dowmwhenithe relay is enersized: from contactiid lto contactlifi In the-case of :relayr 52, whentiteisw energized, the circuit -wilh bee made: at contacts: 58; 60:: andl 6| and" simultanenusllo openedi atscontaot- GT; 63 and 61; and thiszc-cmditiomwillf. be maintained as: long as the relay; int-energized: Whom the gized; the rorigihaltconditionishowntih E'igi 3 will reounr; Referringato Fig; i stagerl; corresponds torrstage Ilofr; 2E maiioperatiomof and fiiniti'ally represents non-- th'e refrigeratorcwith frost con tinually: accumulatiiigroni the evaporatorunit 2 At this time thermistor: elements'U and c are in; control; of? the; Wli'eatston'e bridge, as can i be readilyrseenzby tracing thatch-suits. Th'e end' of stage Ii occurs? whem frostr reacheselement b thereby settingmwthetrequired temperature-dir f erencer betweens, elements":- b" andl 0 and 7 when" thermost'att 3M stops calling for cold, thereby closingxsolenoidivalve 3.65 r and deenergizing relay 40. to; close: switch; am the bridge: as; bBfiJlBZ. Asathepbridgew is lnow 'un balanced; the: various: relays controlledi by: the output: on amplifienw 3i are actuated-, these being releysr I 0 'y, 32!, and) 52: Arttthis point; it will be? convenient to: represent the: various stagesa oientlyrlarge asi stage-r11; .aithoughtsactually: stage II ,1 represents: only; the: very? briefs! period. when =1 changesvocoun' due to: this; unbalance. In: thisr stage II, stepping: relay ll)- will switclrrth'eicona trol from elementsrb andzfcf" to eiementswa' and bf stepping; relay. 3! willi starti the 11681912421 5 relaygaSOt-willcutgofi power. toytliermostatxfll; ale though thezleadz to ztheath'ermostatt has also: been: openedgat contact 3.1 oirsteppihgarelay il 2": (the reasonrforithiswill be showmbelow); ordinary rela3lt=52; is energized to .open

atrcontacts fl; 66 and?- Mvand-i simultaneouslycloseethecircuits-at; oontacts 58c. 60:; and 1 G and ordinary; rela w" rcmainstdeenergized" so" that switch: arm? 41H remains'iclosed; Stage;III ofFig.

thas now bee'unr. Tihe oontrolhavingbeen-trans 1 ferred-to elementsra and-572K, both-of which are innconta'ctwwithether-ice formation-on the evapo= rator unit, the temperature;(inference:between these -twoi-:unitsle is-tadjvustedt by variable resistor 59: sozthatthescurrentrtflowtacrossr'thez bridge is reduced sufiioiently to fdeenergize the :vario'us-zrelaysr. Therefore; the? contacts tOf- 0I'di'na1y;' relaysas 50;;and.52 returmtostheir.originalrpositionsshown in; F 3 Steppingg relays I 0;! and t3 2.

toitheir original position: Th'eheater 42 f therefore, continues-to, defrostias .it... is supplied. from power; terminals. l3., througnswitch. arm. 2.11,

eater. As .theh'eat is. exteris "warmed up very much. more element" a; and" the' temperature' nal element; b? quickly than? dlfl'renoe rapldlir risestato'rthe; point vrliererthee cause aetuati'onthereot-i Tfiis is tlre endiof stage' different; and; system on control" their-r switch arms" relay: is deenere 411;" and thus: energize the;- various: circuits;

how-ever; since it :requires; at

has operated; moreover, since ordinary relay 50.

is also actuated at this time, contact 56 is' engaged by arm 5|, the thermostat is still cutout of the circuit and heater 42' is still on, its circuit extending through contact 56, arm 5|, contact 3i, arm 21 out to terminals. i3. This 'condition (stage IV) continues during the rest of the defrosting operation. As the ice melts from around the unit 2, element a finally also warms up to the point where the temperature difference between it and element b cycle has now been completed.

It will be noted that in Fig. 3, when stepping switch 32' is energized, it not only changes switch 27 from its upper to its lower position, butalso similarly changes switch 3Q from initial engagement with its upper contact to engage-' fication so that the current output-of the ampli- 1 fier during this stage remains of the same order of magnitude as before. The action is as follows: During stage I, the circuit is as shown in Fig. 3, from the left side of the bridge through the upper contact of switch 39, through contact 63 to input A of the amplifier switch, which is the more sensitive stage; during stages'II and III, through lower contact of switch 39 direct to input 13, cutting out input A and thus attaining less sensitivity; during stage IV from left side of bridge through wire 65 tocontact 6|, again to input B; and during stage V same as stage I. The above arrangement may not be needed in all installations, and if found not to be necessary, switch 38 and contacts GI and 53 may be omitted and the circuit to the amplifier may he as shown at 30 in Fig. 1.

It will be understood that in'some situations the use of thermistors a and b as well as a and c for terminating defrosting can be used for either means of heating, although the above-described combination is usually the best for each method of heating under the circumstances ordinarily encountered. It will also be understood that is insufficient to hold. ordinary relays 50 and 52. This is the end of"- stage IV and the beginning of stage V. When this occurs, relays 5! and 52 are deenergized and' return to the position shown in Fig. 3. One full during the'norrnal operation stage of the refrigorator unit, represented by stages I andIII in Fig. 2 and by stages I and V in Fig. 4, the re frigerator thermostat is alternately operating and releasing relay 4!] (or 4B) and that the above described current curves ,of Figs. 2 and 4. ignore the periods when the ther'mos'tatcauses the bridge to be deenerg'iz ed since initiation of defrosting cannot occur during such periods,

However, this only delays initiation of defrosting for a relatively short time during which comparatively little additional frost can form, and once the defrosting action is initiated, the thermostat is no longer in control until defrosting is completed.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.

I claim:

1. The method of automatically defrosting of a refrigerator cooling unit upon which ice tends to form which comprises detecting the temperature difference between a first locus removed from the surface of said cooling unit by the maximum distance to which frost is permitted to extend and a second locus, further from said cooling unit, converting said temperature difference into a first electrical parameter; initiating defrosting action when said nrst electrical parameter attains a predetermined magnitude representing the extension of frost formation to said first locus; detecting the temperature difference between a locus immediately adjacent the surface of said cooling unit and a more remote location from'said cooling unit; converting the last said detected temperature difference into a second electrical parameter; transferring control of the defrosting action from said first electrical parameter to said second electrical parameter; ceasing said defrosting action when. said second electrical parameter attains a predeter- .5 mined magnitude representing elimination of frost from said surface; and reestablishing the initial control conditions.

2. The invention defined in the preceding claim wherein the step of defrosting the cooling unit is performed by heating it from within,

the step of defrosting the cooling unit is performed by heating it from without, said second temperature difference being taken between said locus adjacent the cooling unit and said first locus.

4. The invention according to claim 3 includ ing the step of deenergizing said defrosting control system whenever the refrigerator temperature is above a predetermined value.

5. An automatic defrosting system comprising three temperature-sensitive electrical elements, one located near a surface upon which frost tends to form, the second at the limit of allowable deposit of frost on said surface and the third beyond the reach of frost formation von said surface, said elements being of such nature that they change an electrical characteristic as a function of temperature, a circuit for balancing said electrical characteristics of said second and third elements, when their tempera- =ture difference is within a predetermined range,

circuit means controlled by the degree of unbalance produced when their temperature difference exceeds a predetermined amount to initiate defrosting action and also to transfer con- ,trol of defrosting termination to a circuit for balancing the electrical characteristics of the first and third elements, and means controlle" by said last circuit for restoring the Original circuit condition upon termination of defrosting.

6. In combination with a refrigerator enclosure having a cooling element therein and a thermostatic control system for maintaining a predetermined refrigerator temperature, an automatic defrosting system for said freezing element comprising a first temperature-sensitive electrical element located at a predetermined distance from a surface of said cooling unit upon which frost tends to form; a second temperature-sensitive electrical element located at a further distance from said freezing unit than said first elemerit; a bridge circuit including both said elements for balancing respective electrical characteristics thereof; relay means connected in said bridge circuit; defrosting means for said cooling unit; control means for said defroster means actuated by said relay means upon attainment of a predetermined difference in electrical characteristic between said temperature-sensitive elements; a third temperature sensitive element located adjacent the surface of said cooling unit; a bridge circuit including said last temperaturesensitive element and a temperature-sensitive element more remote from the surface of said cooling unit; switching means controlled by said relay means for transferring control of said defrosting means from said first bridge to said second bridge; means controlled by a predetermined value of the output of said second bridge for restoring the initial circuit condition.

'7. In combination with a refrigerator enclosure having a freezing element therein, an automatic defrosting system comprising a first temperature-sensitive electrical element located at a predetermined distance from a surface of the freezing unit upon which frost tends to form; a second temperature-sensitive electrical element located adjacent said surface, said elements being capable of influencing an electrical parameter of an electrical circuit of which they are a part; first and second electric means including each said element respectively; defrosting control means for controlling defrosting of said surface; circuit connections between said respective electric circuit means and said defrosting control means, whereby a predetermined temperature difference between said first temperature-sensitive element and the refrigerator enclosure remote from said freezing element changes the circuit parameters to actuate said defrosting means to initiate defrosting action; switching means actuated by the change in electrical parameters resulting from said predetermined difference for transferring control of said defrosting means from said first temperature-sensitive element to said second temperature-sensitive element; circuit connections between said second temperature-sensitive element and said defrosting control means for terminating the defrosting when a predetermined temperature difference between said second temperature-sensitive element and the refrigerator enclosure is attained; and means for restoring the initial circuit connections between said first temperature-sensitive element and said first electrical circuit upon termination of said defrosting action.

MINORU FUJII.

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